1,721,025 research outputs found
Spreading of correlations in a quenched repulsive and attractive one-dimensional integrable system
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Quenched dynamics and spin-charge separation in an interacting topological lattice
Full text linkWe analyze the static and dynamical properties of a one-dimensional topological lattice, the fermionic Su-Schrieffer-Heeger model, in the presence of on-site interactions. Based on a study of charge and spin correlation functions, we elucidate the nature of the topological edge modes, which, depending on the sign of the interactions, either display particles of opposite spin on opposite edges, or a pair and a holon. This study of correlation functions also highlights the strong entanglement that exists between the opposite edges of the system. This last feature has remarkable consequences upon subjecting the system to a quench, where an instantaneous edge-to-edge signal appears in the correlation functions characterizing the edge modes. Besides, other correlation functions are shown to propagate in the bulk according to the light cone imposed by the Lieb-Robinson bound. Our study reveals how one-dimensional lattices exhibiting entangled topological edge modes allow for a nontrivial correlation spreading, while providing an accessible platform to detect spin-charge separation using state-of-the-art experimental techniques
Dynamics and correlations in Motzkin and Fredkin spin chains
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Magnetic phase transition in coherently coupled Bose gases in optical lattices
Full text linkWe describe the ground state of a gas of bosonic atoms with two coherently coupled internal levels in a deep optical lattice in a one-dimensional geometry. In the single-band approximation this system is described by a Bose-Hubbard Hamiltonian. The system has a superfluid and a Mott insulating phase that can be either paramagnetic or ferromagnetic. We characterize the quantum phase transitions at unit filling by means of a density-matrix renormalization-group technique and compare the results with a mean-field approach and an effective spin Hamiltonian. The presence of the ferromagnetic Ising-like transition modifies the Mott lobes. In the Mott insulating region the system maps to the ferromagnetic spin-1/2 XXZ model in a transverse field and the numerical results compare very well with the analytical results obtained from the spin model. In the superfluid regime quantum fluctuations strongly modify the phase transition with respect to the well-established mean-field three-dimensional classical bifurcation
Quantum bright solitons in the Bose-Hubbard model with site-dependent repulsive interactions
Full text linkWe introduce a one-dimensional spatially inhomogeneous Bose-Hubbard model (BHM) with the strength of the onsite repulsive interactions growing, with the discrete coordinate zj, as |zj|α with α>0. Recently, the analysis of the mean-field (MF) counterpart of this system has demonstrated self-trapping of robust unstaggered discrete solitons, under the condition α>1. By using the numerically implemented method of the density matrix renormalization group, we demonstrate that, in a certain range of the interaction, the BHM also features self-trapping of the ground state into a soliton-like configuration, at α>1, and remains weakly localized at α<1. An essential quantum feature found in the BHM is a residual quasi-constant density of the background surrounding the soliton-like peak in the ground state, while in the MF limit the finite-density background is absent. Very strong onsite repulsion eventually destroys soliton-like states, driving the system, at integer densities, into the Mott phase with a spatially uniform density
Familial Alzheimerâ€TMs Disease presenilin mutants reduce calcium levels of intracellular stores. A critical revaluation of the “calcium overload†hypothesis
No full textBackground: Presenilin-1 and -2 (PS1 and PS2) mutations, the major cause of Familial Alzheimer’s Disease (FAD), have been causally implicated in the pathogenesis of neuronal cell death through a perturbation of cellular Ca2+ homeostasis. We have recently shown that, at variance with previous suggestions obtained in cells expressing other FAD-linked PS mutations, PS2-M239I and PS2-T122R cause a reduction and not an increase in cytosolic Ca2+ rises induced by Ca2+ release from stores (1,2). Objective(s): In this study we aim at investigating whether other FAD-linked PS mutations induce a similar dysregulation of Ca2+ homeostasis. Methods: Different cell models have been used: human fibroblasts from controls and FAD patients, cell lines (SH-SY5Y, HeLa, HEK293, MEFs) and rat primary neurons expressing a number of PS mutations, e.g. P117L, M146L, L286V, and A246E in PS1 and M239I, T122R, and N141I in PS2. The effects of FAD-linked PS mutations on cytosolic Ca2+ changes have been monitored either by using fura-2 or recombinant cytosolic aequorin as the probe. Results: Independently of the cell model or the employed probe, the cytosolic Ca2+ increases, caused by agonist stimulation or full store depletion by drug treatment, were reduced or unchanged in cells expressing the PS mutations. Using aequorins, targeted to the endoplasmic reticulum or the Golgi apparatus, we here show that FAD-linked PS mutants lower the Ca2+ content of intracellular stores. The phenomenon was most prominent in cells expressing PS2 mutants, and was observed also in cells expressing the non-pathogenic, “loss-of-function” PS2-D366A mutation. Conclusions: Taken as a whole, our findings, while confirming the capability of presenilins to modify Ca2+ homeostasis, suggest a re-evaluation of the “Ca2+ hypothesis” in AD and a new working hypothesis is presented.
1. Zatti G, Ghidoni R, Barbiero L, Binetti G, Pozzan T, Fasolato C, Pizzo P. (2004). Neurobiology of disease, 15, 269-278.
2. Giacomello M, Barbiero L, Zatti G, Squitti R, Binetti G, Pozzan T, Fasolato C, Ghidoni R, Pizzo P. (2005). Neurobiology of disease, 18, 638-648
Localized-interaction-induced quantum reflection and filtering of bosonic matter in a one-dimensional lattice guide
Full text linkWe study the dynamics of quantum bosonic waves in a one-dimensional tilted optical lattice. An effective spatially localized nonlinear two-body potential barrier is set at the center of the lattice. This version of the Bose-Hubbard model can be realized in atomic Bose-Einstein condensates, with the help of localized optical Feshbach resonance, controlled by a focused laser beam, and in quantum optics, using an arrayed waveguide with selectively doped guiding cores. Our numerical analysis demonstrates that the central barrier induces anomalous quantum reflection of incident wave packets, which acts solely on bosonic components with multiple onsite occupancies, while single-occupancy components pass the barrier, allowing one to distill them in the interaction zone. As a consequence, in this region one finds a hard-core-like state, in which the multiple occupancy is forbidden. Our results demonstrate that this regime can be attained dynamically, using relatively weak interactions, irrespective of their sign. Physical parameters necessary for the experimental implementation of the setting in ultracold atomic gases are estimated
Reduction of Ca2+ stores and capacitative Ca2+ entry is associated with the familial Alzheimer's disease presenilin-2 T122R mutation and anticipates the onset of dementia
Full text linkMutations in the presenilin genes PS1 and PS2, the major cause of familial Alzheimer's disease (FAD), are associated with alterations in Ca2+ signalling. In contrast to the majority of FAD-linked PS1 mutations, which cause an overload of intracellular Ca2+ pools, the FAD-linked PS2 mutation M239I reduces Ca2+ release from intracellular stores [Zatti, G., Ghidoni, R., Barbiero, L., Binetti, G., Pozzan, T., Fasolato, C., Pizzo, P., 2004. The presenilin 2 M239I mutation associated with Familial Alzheimer's Disease reduces Ca2+ release from intracellular stores. Neurobiol. Dis. 15/2, 269-278]. We here show that in human FAD fibroblasts another PS2 mutation (T122R) reduces both Ca2+ release and capacitative Ca2+ entry. The observation, done in two monozygotic twins, is of note since only one of the subjects showed overt signs of disease at the time of biopsy whereas the other one developed the disease 3 years later. This finding indicates that Ca2+ dysregulation anticipates the onset of dementia. A similar Ca2+ alteration occurred in HeLa and HEK293 cells transiently expressing PS2-T122R. Based on these data, the "Ca2+ overload" hypothesis in AD pathogenesis is here discussed and reformulated
Bound state dynamics in the long-range spin- 1⁄2 XXZ model
Full text linkExperimental platforms based on trapped ions, cold molecules, and Rydberg atoms have made possible the investigation of highly nonlocal spin-1/2 Hamiltonians with long-range couplings. Here, we study the effects of such nonlocal couplings in the long-range spin-1/2 XXZ Heisenberg Hamiltonian. We calculate explicitly the two-spin energy spectrum, which describes all possible energetic configurations of two spins pointing in a specific direction embedded in a background of spins with opposite orientation. For fast decay of the spin-spin couplings, we find that the two-spin energy spectrum is characterized by well-defined discrete values, corresponding to bound states, separated by a set of continuum states describing the scattering region. In the deep long-range regime instead, the bound states disappear as they get incorporated by the scattering region. The presence of two-spin bound states results to be crucial to determine both two- and many-spin dynamics. On one hand, radically different two-spin spreadings can be observed by tuning the decay of the spin couplings. On the other hand, two-spin bound states enable the dynamical stabilization of effective antiferromagnetic states in the presence of ferromagnetic couplings. Finally, we propose a novel scheme based on a trapped-ion quantum simulator to experimentally realize the long-range XXZ model and to study its out-of-equilibrium properties
Haldane topological orders in Motzkin spin chains
Full text linkMotzkin spin chains are frustration-free models whose ground state is a combination of Motzkin paths. The weight of such path contributions can be controlled by a deformation parameter t. As a function of the latter, these models, besides the formation of domain wall structures, exhibit gapped Haldane topological orders with a constant decay of string order parameters for t<1. A behavior compatible with a Berezinskii-Kosterlitz-Thouless phase transition at t=1 is also presented. By means of numerical calculations we show that the topological properties of the Haldane phases depend on the spin value. This allows one to classify different kinds of hidden antiferromagnetic Haldane gapped regimes associated with nontrivial features such as symmetry-protected topological order. On one hand, our results allow one to clarify the physical properties of Motzkin frustration-free chains, and on the other hand, suggest them as an interesting and paradigmatic class of local spin Hamiltonians
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